1. Field of the Invention
The present invention relates to a once-through vapor generator, and in particular to a novel start-up bypass system for a once-through vapor generator.
A typical once-through vapor generator, of the type to which the present invention pertains, will include an inlet end and an outlet end, with a plurality of heat transfer surfaces between the ends. As a general rule, these will include an economizer pass, furnace passes defining the high temperature radiant heat transfer portion of the generator, a reheater, and primary and finishing superheating passes, the outlet end of the generator being connected to a suitable point of use such as a high pressure steam turbine. The exhaust flow from the turbine or turbines is transmitted to condensing means, a deaerator, and from there through heat recovery surfaces to the inlet end of the generator.
During start-up of the vapor generator, the low enthalpy fluid cannot be handled by the high pressure turbine, and for this reason, the generator usually is provided with a bypass system to recirculate the flow until the flow is at the enthalpy level required by the turbine. It is known to transmit this flow to heat recovery surfaces where it is passed in heat exchange with the feed flow to the vapor generator inlet end. It is also known to position a flash tank or separator in the bypass system designed to separate the flow entering the bypass system into vapor and liquid streams and to transmit the vapor stream back to the main flow path for warming and rolling the high pressure turbine. Other uses are known for the bypass flow, including turbine gland sealing, and pegging the deaerator.
Depending upon the design of the vapor generator, there may be no flow during at least the initial stages of start-up through certain surfaces, for instance the reheater surfaces of the generator, and perhaps even through primary and finishing superheating surfaces. These surfaces usually are positioned in lower temperature or convection heating zones, so that during the initial stages of start-up, cooling of the surfaces is not necessary. Accordingly, the bypass system usually is connected to the main flow path upstream of at least the finishing superheater surface. This has the advantage that the vapor flow returned to the main flow path from the bypass system flash tank or separator can be subjected to further heating and superheating for earlier warming and rolling of the high pressure turbine, reducing the start-up period.
In a once-through boiler, prior to firing, sufficient flow is required through the boiler pressure parts which are exposed to high gas temperatures during start-up. With smooth furnace tubes, a minimum flow at 25 percent of full load flow generally represents a good balance between furnace tube cooling requirements over the load range and furnace enclosure pressure drop. In the low load range, tube fluid mass flow cannot be reduced appreciably without the possibility of "pseudo film boiling," a condition much like that known as "departure from nucleate boiling" (DNB) at subcritical pressures. Like DNB, pseudo film boiling represents a sudden deterioration of heat transfer at the internal tube surface, which results in unacceptably high levels of tube metal temperatures and must be avoided under all operating conditions.
The multi-lead ribbed tube has found broad acceptance as a tool to prevent DNB in 2400 psi boilers and has proved very effective in preventing pseudo film boiling at supercritical pressures. Ribbed tubing use enables reduction of the minimum flow to 15 percent of full load flow of 3500 psi boilers. Lowering the minimum flow means reduced start-up time, reduced heat loss to the condenser, and reduced auxiliary steam and auxiliary power requirements during start-up. The lower minimum flow also offers the ability to operate the boiler over a wider load range, 20 to 100%, without going on the bypass system. This wider load range can be handled for most domestic coals without oil support and with reasonable net plant heat rates.
On many units, the high pressure turbine has been subjected to temperature dips during start-ups when the steam temperature dropped while ramping throttle pressure. The temperature control problems during start-up stem from the practice of ramping superheater pressure to full operating pressure at relatively low loads. While ramping pressure, superheater flow is raised from approximately 7 to 25%, and simultaneously, flow is shifted from the flash tank to the normal path through the boiler. All of this happens at very low loads where long time lags and large changes in fluid storage and heat storage in the boiler preclude effective steam temperature control.
Developments have taken place to overcome these shortcomings; specifically, to reduce minimum start-up flow, simplify and speed up start-up, permit controlled shutdowns, provide positive control of steam conditions, and enable quick load changes over a wide load range. Some of these developments are; the use of internally ribbed tubes, the concept of dual pressure operation (i.e., capability to operate at variable throttle pressure while maintaining fluid pressure in the economizer, boiler enclosure, and primary superheater), capability of variable throttle pressure operation over a wide load range, and use of saturated steam for attemperation of main and reheat steam during start-up and at low loads.
One problem experienced with conventional bypass systems is that as the vapor generators become larger in size, and of much larger capacity, the bypass systems of necessity must be designed to handle ever greater quantities of flow; that is, the 30% minimum flow becomes increasingly greater in terms of total mass flow. The flash tanks or separators positioned in the bypass systems also must be capable of handling the increased flows as capacities of the generators increase, and since these flash tanks or separators are heavy walled vessels designed to withstand high pressures, and temperatures, it is apparent that the separators or flash tanks become major items in the capital costs of the generator, particularly in the cost of the bypass system. It is known, to use a plurality of smaller sized flash tanks or separator vessels in place of one large very heavy walled vessel. However, whether one or several vessels are used, the expense of this part of the system is high and can be out of proportion when compared with the remainder of the system.
A further disadvantage experienced with conventional bypass systems concerns switch-over of flow from the bypass system to the main flow path at the point in the start-up period when the bypass system is isolated from the flow. Although the bypass systems and flash tanks or separators can be designed to handle flows up to full operating pressures and temperatures in a once-through vapor generator, which may be in the order of 3,600 psi and about 1,100.degree. F., respectively, economics (as discussed above) dictate that the bypass system be designed for and utilized up to only about 1,000 psi, at which time or point in the start-up period the flow is switched over to the main flow path. Since the bypass system is positioned upstream of one or more of the superheating sections, for shorter start-up time, there usually is insufficient surface upstream of the bypass system to produce a fully vaporized flow at the normal switch-over pressure of about 1,000 psi, at this point in the start-up period. The result is that the surfaces downstream of the bypasss system, which prior to switch-over, will have received a vapor flow from the flash tank or separator, will now receive a vapor-liquid mixture flow from the upstream surfaces, resulting in a temperature drop in the surfaces downstream of the bypass system and an undesirable temperature shock to these surfaces.
2. Description of Prior Art
U.S. Pat. No. 3,529,580 (Stevens) describes a once-through vapor generator comprising a main flow path and a bypass system which includes a first and second conduit means connected to the main flow path, a flash tank means, vapor and liquid return lines from the flow tank, and the second conduit means leading to the heat recovery surfaces and including a valve means therein so as to apportion the flow from said main flow path between the first and second conduit means.
U.S. Pat. No. 3,954,087 (Stevens et al) discloses an apparatus and method of start-up of a subcritical and supercritical once-through vapor generator. This system comprises a plurality of separators which are capable of handling full load flow and auxiliary flow paths, one to the condenser and the other to the main flow path between the condenser and vapor generator.
Other revelant prior art consists of U.S. Pat. Nos. 3,338,053 and 3,338,055 (Gorzegno, et al). Disclosed is a start-up apparatus and methods for a subcritical and supercritical vapor generator. This system is comprised of a pressure reducing means in the main flow path between the vapor generating surface and superheaters, a flash tank means connected between the two superheater surfaces, and liquid and vapor bypass conduit means from said flash tank means.
In accordance with the present invention, there is provided a once-through vapor generator comprising a main flow path, heating surfaces, heat recovery surfaces and a bypass system. The bypass system includes a flash tank means sized to handle flow up to at least 25% of full load, a variable superheater bypass stop valve located between the superheating surfaces and sized to handle variable superheating pressure from approximately 15% to full load, and two steam spray attemperators from the flash tank; one to the outlet of the second superheater and the second to the outlet of a reheater located between a first high pressure turbine and a second low pressure turbine.
It is an object of the present invention to overcome the above problems, and in particular to provide a simplified bypass system capable of avoiding the temperature shock experienced in conventional systems during switch-over from the bypass system to main path flow.
It is an object of this invention to maintain minimum flow through the boiler furnace parts exposed to high temperature combustion gases during start-up.
It is an object of this invention to provide means of positive control of steam conditions during start-up and shutdown to suit turbine metal requirements.
It is an object of this invention to recover heat during start-up and low load operation. It is an object of this invention to provide for water cleanup during start-up without delays in boiler and turbine warming operations.
It is the final object of this invention to provide means of operating at variable throttle pressure over the load range while maintaining the necessary supercritical pressure in the furnace circuits.